1. Field of the Invention
[Technical Field]
The present invention relates to a planar light source, a display device, a portable terminal device, and a ray direction switching element, and in particular, to a planar light source that can change an irradiation angle of illumination light, a display device that can change an angle of field using the planar light source, a portable terminal device that uses the display device, and a ray direction switching element that is incorporated in the planar light source.
2. Description of the Related Art
[Background Art]
In accordance with the development of technologies in recent years, a liquid crystal display device (LCD), which is wide in an angle of field, that is, visually recognizable in a wide angle range, has been put to practical use. In addition, a portable information terminal mounted with the LCD is also widely used. In such a portable information terminal, it is desirable that the angle of field of the LCD is wide when a user looks at information displayed on the LCD with other people. On the other hand, in the portable information terminal, the user often does not want other people to peep at displayed information. In such a case, it is desirable that the angle of field of the LCD is narrow. In this way, the angle of field is required to be wide and narrow depending on a state of use of the LCD. Conventionally, LCDs meeting such a demand have been proposed.
FIGS. 24(a) and 24(b) schematically show a first conventional liquid crystal display device that is described in Japanese Patent Laid-Open No. 6-59287. FIG. 24(a) shows the liquid crystal display device at the time when a voltage is not applied thereto. FIG. 24(b) shows the liquid crystal display device at the time when a voltage is applied thereto. As shown in FIGS. 24(a) and 24(b), the first conventional liquid crystal display device includes a liquid crystal panel in which a liquid crystal material (not shown) is sealed by transparent substrates 102 and 108. A polarizing plate 101 is provided on one surface of this liquid crystal panel. On the other surface, a guest host liquid crystal cell 131, in which a liquid crystal material consisting of liquid crystal molecules 131a and elongate pigment molecules 131b are sealed by two transparent substrates 114 provided with transparent electrodes 110 on surfaces thereof, is provided. The pigment molecules 131b have a larger amount of absorption of light in a minor axis direction of the molecules than in a major axis direction thereof. When a voltage is not applied to the guest host liquid crystal cell 131, the liquid crystal molecules 131a and the elongate pigment molecules 131b are arranged to be parallel to the surfaces of the transparent substrates 114 in a longitudinal direction. When a voltage is applied to the guest host liquid crystal cell 131, the liquid crystal molecules 131a and the elongate pigment molecules 131b are arranged to be perpendicular to the surfaces of the transparent substrates 114 in the longitudinal direction. The polarizing plate 101 is provided on a surface on the opposite side of a surface opposed to the liquid crystal panel of the guest host liquid crystal cell 131.
In the first conventional liquid crystal display device constituted in this way, which is described in Japanese Patent Laid-Open No. 6-59287, light in a wide angle range passes through the liquid crystal panel to be made incident on the guest host liquid crystal cell 131. When an image is displayed at a wide angle of field, a voltage is not applied to the guest host liquid crystal cell 131 to make a light absorbing direction of the guest host liquid crystal cell 131 coincident with an absorbing direction of the polarizing plate 101, whereby the light passes through the guest host liquid crystal cell 131 directly. Consequently, it is possible to visually recognize a display screen in a wide angle range.
When an image is displayed at a narrow angle of field, when a voltage is applied to the guest host liquid crystal cell 131, the pigment molecules 131b are arranged to be perpendicular to the surfaces of the transparent substrates 114 in the longitudinal direction, and an angle of incidence of light deviates largely from a direction perpendicular to the surfaces of the transparent substrates 114. This light is absorbed by the pigment molecules 131b and does not pass through the guest host liquid crystal cell 131. Therefore, even if an angle distribution of light made incident on the display device is wide, an angle distribution of emitted light is narrowed by absorption of the guest host liquid crystal. Consequently, it is possible to reduce a size of a visually recognizable display screen.
FIG. 25 is a diagram schematically showing a second conventional liquid crystal display device that is described in Japanese Patent Laid-Open No. 10-197844. The second conventional liquid crystal display device includes a backlight 113, as shown in FIG. 25. A PDLC cell 136, in which a Polymer Dispersed Liquid Crystal (PDLC) layer 111 is sandwiched by two transparent substrates 109, is provided on the backlight 113. A polarizing plate 101 is provided on the PDLC cell 136, and a Twisted Nematic-Liquid Crystal Display (TN-LCD) is provided on the polarizing plate 101. A guest host liquid crystal cell is provided on the TN-LCD, and the polarizing plate 101 is provided on the guest host liquid crystal cell. This guest host liquid crystal cell has the same structure as the guest host liquid crystal cell that is used in the first conventional liquid crystal display device described in Japanese Patent Laid-Open No. 6-59287.
In the second conventional liquid crystal display device constituted in this way, which is described in Japanese Patent Laid-Open No. 10-197844, wide field of view display and narrow field of view display are switched by switching ON and OFF of a voltage applied to the guest host liquid crystal cell. In addition, transmission and reflection of light is switched by switching ON and OFF of a voltage applied to the PDLC cell to adjust brightness of a display screen.
Japanese Patent Laid-Open No. 11-142819 discloses a liquid crystal display device in which a condensing element consisting of a prism sheet and a light scattering element consisting of a PDLC cell are provided between a light source and a liquid crystal panel. Japanese Patent Laid-Open No. 11-142819 mentions that it is possible to switch a narrow angle of field and a wide angle of field by increasing directivity of light with the prism sheet and, then, transmitting or scattering light from the prism sheet with the PDLC cell. In addition, Japanese Patent Laid-Open No. 9-105907 discloses a similar liquid crystal display device in which an optical element consisting of a PDLC cell is provided between a light source and a liquid crystal panel.
On the other hand, conventionally, a high directivity backlight, in which an irradiation range of illumination light is fixed but directivity in a specific direction such as the front direction is improved, has been developed (see, for example, monthly magazine “Display” May 2004, pages 14 to 17). FIG. 26 is a perspective view showing a conventional high directivity backlight 213 described in the monthly magazine “Display” May 2004, pages 14 to 17. As shown in FIG. 26, in this conventional high directivity backlight 213, an LED 201 is arranged in one location where a light guide plate 202 is provided, and a linear micro-prism is arranged concentrically around the LED 201 in the light guide plate 202. A prism sheet 203, in which a prism structure is also arranged concentrically around the LED 201, is arranged on a light emission surface of the light guide plate 202. In addition, a reflection sheet 204 is arranged on a surface on the opposite side of the surface of the light guide plate 202 on which the prism sheet 203 is provided.
Exit light from the LED 201 is made incident on the light guide plate 202 and emitted radially along the surface of the light guide plate 202 by the linear micro-prism formed in the light guide plate 202. At this point, the LED 201 is arranged in one location of the light guide plate 202, and a longitudinal direction of the linear micro-prism formed in the light guide plate 202 is arranged to be substantially perpendicular to the LED 201. Thus, even if light guided through the light guide plate 202 hits the linear micro-prism, the light is not deflected in the longitudinal direction of the linear micro-prism but travels linearly and radially around the LED 201. The light emitted from the light guide plate 202 is refracted by the prism sheet 203 and deflected in a vertical direction with respect to the light emission surface of the light guide plate 202. Consequently, a high directivity backlight, in which directivity is improved two-dimensionally in a front direction, is realized.